The main characters are a boy named Yoshimori Sumimura and a girl named Tokine Yukimura. Most of the first episode is set when they are aged 9 and 11, respectively, but by the end, it's flashed forward to when they're 14 and 16, which is when the series will be set henceforth. Anyway, the two of them are Kekkaishi, which means "barrier magic users". They use their kekkai magic to destroy demons called Ayakashi (which come in lots of different types). The Ayakashi are drawn to Karasumori Academy, where there is a great deal of spiritual energy, which makes the Ayakashi much more powerful and dangerous. The Sumimura and Yukimura clans are both descended from the clan of Tokimori Hazama, who had served the Karasumori clan (apparently the source of the power that attracted Ayakashi) 500 years ago, as a Kekkaishi. Though as more and more Ayakashi came, Hazama was overworked, and fell ill, so that for three days he couldn't battle. When he recovered, he found the Karasumori clan had all been killed by Ayakashi. The source of the Karasumori clan's power is now enshrined deep underground, below the Academy (which was built over the site where Karasumori castle once stood), but as I said, Ayakashi continue to be drawn to the power. So there have been many generations of Kekkaishi protecting the site. Yoshimori and Tokine are each the 22nd Kekkaishi of their respective families.

The two young Kekkaishi each have demon dogs, which can help them locate Ayakashi. Yoshimori's is named Madarao, and Tokine's is named Hakubi. Yoshimori is trained by his grandfather, Shigemori Sumimura, and Tokine is trained by her grandmother, Tokiko Yukimura (the two of them were the 21st Kekkaishi of their families). Shigemori and Tokiko often argue and even fight each other, each claiming their clan is the true successor to Hazama, and they don't want Yoshimori and Tokine to have anything to do with each other. However, the kids remain friends, and Yoshimori is determined to protect Tokine, ever since she was injured while protecting him, toward the end of the first episode (five years prior to the main time of the series). He's not as disciplined nor as skilled as she is, but he possesses greater power. Oh, and he seems to have a crush on her, though she still sees him as a little kid, and is often frustrated by the way he acts when they're protecting the site at night. And during the day when they attend Karasumori Academy as students, she usually just ignores him.

Eventually we learn about a group called the Shadow Organization, which deals with all kinds of supernatural stuff such as Ayakashi, though they generally leave Karasumori in the hands of the Kekkaishi. Yoshimori's older brother, Masamori, is head of the Night Troop, which is one branch of the Shadow Organization. He sends one of his people, a boy named Gen Shishio, to Karasumori to assist Yoshi and Tokine, though Yoshi doesn't get along with him well. And Gen isn't too impressed by the Kekkaishi's power at first, himself. He's kind of antisocial as well as feeling superior. We also learn after a bit that he's half Ayakashi. The three of them eventually learn that there's a group called the Kokuboru who are interested in the Karasumori site, and are causing trouble in various ways. They've also created artificial human skin that Ayakashi can wear to appear human. So the guardians of Karasumori have to learn more about these new enemies. The reason the Kokuboru are interested in Karasumori is apparently that they need to aquire power to save the life of their Princess, or something.

We eventually get to know several important members of the Kokuboru, including their leader, Byaku. More important to the Kekkaishi is one named Kaguro; for a reason I don't want to spoil, Yoshimori becomes obsessed with defeating him. We also get to see a bunch of characters from the Night Troop, who eventually come to Karasumori to help the Kekkaishi defend the site against the Kokuboru, who stage a couple of different massive attacks. The main member of the Night Troop that we get to know is Sen. There's also a woman named Atora who had been Gen's trainer sometime prior to his joining the Kekkaishi. Toward the end of the series, Yoshimori, Tokine, and Sen all go into the world of the Kokuboru, and are later followed by Masamori and the Night Troop. Oh, and I should also mention there's an old man named Heisuke Matsudo, who knows a lot about Ayakashi and such, and who is an associate of Shigemori. Matsudo has a demon servant who he calls Miss Kagami. We eventually learn that Matsudo has a very old grudge against Byaku.

Well, there are various other characters in the series, though mostly they don't seem very important, such as students at school, most of whom know nothing at all about all the supernatural stuff that goes on there at night. Though there is a girl named Yurina Kanda who can see spirits, which always freaks her out. But she knows that Yoshi fights them, so she talks to him about it sometimes. Yuri also seems to have a crush on Yoshi, which her friends tease her about. Anyway, I've said all I want to about the plot. I don't really feel like I've spoiled anything, and I certainly don't want to say how it all ends. It's a fairly fun show, I guess, though not really great. I like the animation style, there are cute girls, it can be funny and silly show, as well as having some reasonably interesting magical battles and stuff. I don't feel like I ever learned as much as I wanted to about either the Shadow Organization or the Kokuboru, though. The show probably could have done with less filler episodes, but even they were okay, I guess. Can't really think what else to say.